As a production method of seamless tubes by hot working, the Mannesmann-mandrel mill tube-making process is widely adopted. In this tube-making process, a heated round billet is pierced with a piercing machine for producing a thick-wall hollow shell, thereafter with a mandrel bar, which is a rolling tool for constraining the inner surface of the hollow shell, being inserted thereinto, the hollow shell is passed through a mandrel mill including a plurality of stands each comprised of grooved rolls located opposite one another, thus being rolled to a light-wall tube blank. The tube blank obtained by mandrel mill rolling is heated again as required, which is followed by a diameter adjusting rolling with a stretch reducer or sizer for finishing the outside diameter to an end product diameter.
Generally, the mandrel bar for use in mandrel mill rolling is manufactured by using a round bar as a starting material that is a hot working tool steel, such as SKD6 or SKD61 as being defined by the JIS standard, and an appropriate machining, quenching and tempering are applied to the round bar. Normally, on the surface of a mandrel bar, a lubricating film essentially consisting of a solid lubricant is formed in advance in order to reduce the frictional force that is caused by contact with the inner surface of hollow shell during rolling.
However, since the surface of the mandrel bar during rolling is exposed to a tremendously high interfacial pressure and thermal load, it is not easy to ensure a stable lubrication condition, although the lubricating film is formed. Therefore, while the mandrel bar being repeatedly used, the surface and base metal thereof are susceptible to wear, seizure, surface deterioration, and cracks, resulting in the shortening of its service life.
From such circumstances, in a recent mandrel mill rolling, a mandrel bar whose base metal is subjected to Cr-plating to form a hard Cr-plating film on the base metal surface (hereinafter referred to as “Cr-plated mandrel bar”) is used. The Cr-plated mandrel bar is protected by a Cr-plating film which is excellent in wear resistance, whereby it provides an excellent durability even in a repetitive use in a mandrel mill rolling, being hardly damaged.
However, the Cr-plating film of the Cr-plated mandrel bar may be peeled off, depending upon use conditions, and once it happens, the base metal comes out in the peeled-off area to thereby get damaged so that it becomes unable to use further the Cr-plated mandrel bar. In order to prevent the mandrel bar from being damaged due to such peeling-off of the Cr-plating film, various proposals for improving service life of mandrel bar have been made.
For example, Japanese Patent Application Publication No. 08-071618 proposes a mandrel bar which has a Cr-plating film with an average thickness of 1 to 100 μm on its base metal surface, and, on the surface of this Cr-plating film, an oxide scale layer essentially consisting of Cr with a thickness of 0.1 to 10 μm. According to the same patent document, by defining the thickness of the Cr-plating film, the peeling-off of the film due to the internal residual stress in the film itself is suppressed, while seizure of the mandrel bar being prevented, and in addition to this, by forming oxide scale on the Cr-plating film and defining the thickness thereof, the lubricity is improved, while allowing the adherence of the Cr-plating film to be improved by action of mutual diffusion of Cr in association with heating at the time of scale formation, whereby the service life of the mandrel bar can be extended.
Japanese Patent Application Publication No. 2001-001016 proposes a mandrel bar which has a Cr-plating film with a thickness of 60 to 200 μm on its base metal surface. According to the same patent document, by defining the thickness of the Cr-plating film, the peeling-off of the film due to the internal residual stress in the film is suppressed, while allowing seizure of mandrel as being caused by the wear of the film to be prevented, whereby the service life of the mandrel bar can be extended.
International Application Publication No. WO2004/108311 proposes a mandrel bar which has a Cr-plating film formed on its base metal surface, wherein its center line average roughness Ra both in an axial direction and a circumferential direction is 1.0 to 5.0 μm, and its maximum depth roughness Rv both in an axial direction and a circumferential direction is 10 μm or over. According to the same patent document, by defining a surface condition of the Cr-plating film both in an axial direction and a circumferential direction, the lubricant can be sufficiently preserved on the film surface during rolling to thereby prevent the seizure of the mandrel bar, and extend the service life of the mandrel bar.